1. Field of the Invention
The invention relates generally to anisotropic plasma etching and more particularly to a method for precision low-pressure plasma etching of polycrystalline silicon semiconductor devices.
2. Description of the Prior Art
Systems using wet etchants for precision etching of semiconductors in microlithography applications have been previously described. However, the use of wet etchants frequently results in loss of precision in replication of the image due to such factors as undercutting, that is, lateral etching underneath photoresist mask edges, attack by the etchant solution on substrate and photoresist materials, poor control of the profile, loss of adhesion resulting in photoresist lifting, and swelling of the photoresist material. Recently, dry etching techniques have been proposed in which a chemically reactive gas is infused in the presence of a cold plasma or glow discharge to selectively remove areas exposed by the photoresist layer. Dry etching has replaced wet chemical etching in selected semiconductor device fabrication steps because of its capability of accurately replicating photoresist images into underlying films anisotropically, i.e., providing vertical or controllably sloped substantially vertical sidewalls. This capability is believed to be due to a combination of isotropic chemical reactions and anisotropic (directional) physical interactions to the substrate surface. These electric fields accelerate positive ions of a reactive gas which bombard the horizontal substrates normally, thus enhancing the etching rate of horizontally bombarded surfaces with respect to the vertical surfaces which are not ion bombarded.
Dry etching technology is generally described in terms of plasma etching, having chamber pressures of 25-65 Pa (200-500 .mu.m) or greater with wafers loaded on a lower grounded electrode, or by the use of reactive ion etching (RIE) apparatus employing pressures of 0.13-13 Pa (1-100 .mu.m) with wafers loaded on the lower powered electrode. Due to the anisotropic ion bombardment effects, RIE conditions typically yield a lower degree of undercutting or lateral etching underneath the mask. Plasma etching has heretofore continued to present problems with undercutting or lateral etching underneath a mask, attributed to insufficient differentiation in etching rates for horizontal and vertical surfaces. See, for example, S. E. Bernacki, Low Pressure Anisotropic Plasma Etching of Doped Polysilicon in CCl.sub.4, Electrochemical Society Inc., May 9-14, 1982 p. 344, which is herein incorporated by reference. However, it has been found possible to obtain anisotropic etching under plasma etching conditions by exploiting chemical reactions occurring in the plasma to form a protective polymer film on the vertical edges of the sidewalls of the mask and polycrystalline silicon (hereinafter also called "polysilicon") films.